The Doppler dimming technique is used for the first time to study ultraviolet polar plumes in the height range of 1.05-1.35 R$_{solar}$, using observations from the spectrometer SUMER on the Solar and Heliospheric Observatory. It is found that, contrary to a number of published suggestions, outflow velocities in the plumes exceed those in the interplume regions. Plume velocities are in excess of 60 km s$^{-1}$ and are approximately constant throughout this height region. They tend to converge with the velocity of the accelerating interplume material at some height above our region of study. The analysis suggests that plume material makes a substantial contribution to the total line of sight, favoring either a ``curtain'' model for plumes or a chance alignment of a number of elementary cylindrical plumes. The intrinsic local density of plume material is some 20%-50% in excess of the interplume regions. Estimation of the total mass outflow indicates that approximately half of the fast solar wind at 1.1 R$_{solar}$ arises from plumes, with the remainder from interplume material. This result validates the published electron temperature profile of David et al. for the fast wind onset, which had been questioned over the suggestion that the flow velocity might be negligible in solar plumes.